The surface gravities of cool dwarf stars revisited

On the base of high-resolution spectra and standard model atmosphere analyses we propose to employ the pressure-broadened Mg Ib lines to derive the gravity parameter for F and G stars. These lines are advocated to be a much more robust and reliable tracer compared to the ionization equilibrium of, say, Fe I/Fe II, which is susceptible to overionization effects and uncertainties in the temperature structure of the model atmosphere. It is demonstrated that the strong line method circumvents the long-standing discrepancy (Delta log g similar to 0.5 dex) of the standard F star Procyon, the surface gravity of which is precisely known due to its nearness and binary nature. We also discuss similar effects on other, predominantly metal-poor stars. In fact, many of the F and hotter G stars deviate in the LTE metal abundances of neutral and ionized species by up to 0.2 dex. However, once the surface gravity parameter is fixed, very reliable iron abundances from Fe II can be derived as well. As a consequence a number of stars considered to be standards will require revised stellar parameters in future analyses. This will have some impact on stellar distances, ages, and galactic evolution in particular.